The invention relates to a method for monitoring an electric load element for thermal overloading in a motor vehicle.
In motor vehicles a battery, charged, for example, by a generator (dynamo), stores electrical energy. When the drive motor of the motor vehicle is switched off some electric consumers are supplied by the energy stored in the battery alone. These consumers reducing the stored energy of the battery include, for example, safety devices, door locks, anti-theft devices or operation of adjusting devices.
The electrical energy stored in the battery (accumulator) has, in addition, to be sufficient after a prolonged stoppage of the drive motor, i.e. without charging by the generator, to allow starting of the drive motor even after several days or even several weeks. Therefore, there must still be sufficient electrical energy stored in the battery even in this case.
In modern motor vehicles there are a large number of electric consumers, for example integrated electric circuits for monitoring functions and control functions, and controllable load elements, such as electric motors for seat adjustments or electric window lifters, and locks, antitheft devices and power semiconductors, causing a reduction in the energy stored in the battery, but which have to be available at least at times as added features. The electric circuits also require continuous electrical energy for monitoring functions and control functions. The energy consumption is noticeably increased owing to the increasingly large number of electric circuits, so the energy reserve of the battery is used up more quickly.
Thermal monitoring of a load element is particularly necessary for load elements, such as electric motors for adjusting devices, which must not exceed a maximum temperature, as otherwise irreparable thermal damage stops the electric motor. The power loss in the armature windings (of the electric motor) occurring during operation of the load element lead to considerable heating of the electric motor.
A method is known from DE 42 16 040 A1 for determining the temperature of the armature windings without a sensor where heating of the electric motor is determined with the aid of the power loss or a variable proportional thereto, the speed of the electric motor. Once a threshold has been reached the current supply to the electric motor is switched off and the electric motor is thus protected against thermal overload.
Determination of cooling behaviour of the electric motor is known from DE 40 38 198 A1. For this purpose, the cooling behaviour over time is detected in a microprocessor and the instantaneous temperature value continuously calculated in the controller by means of an algorithm. The drawback is the high power consumption of the electric circuit and microprocessor for calculating the cooling behaviour in the controller.